1. Field of the Invention
This invention relates to a process for extracting hydrocarbons from the earth. More particularly, this invention relates to a method for recovering viscous hydrocarbons such as bitumen from a subterranean reservoir by injecting steam via a well into the reservoir to lower the viscosity of the hydrocarbon thereby stimulaing production.
2. Description of the Prior Art
In many area of the world, there are large deposits of viscous petroleum, such as the Athabasca and Cold Lake region in Alberta, the Jobo region in Venezuela and the Edna dnd Sisquoc regions in California, U.S.A. These deposits are often referred to as "tar sand" or "heavy oil" deposits due to the high viscosity of the hydrocarbons which they contain. While some distinctions have arisen between tar sands (viscosity between about 10,000 and 100,000 cp @ reservoir temperature) and heavy oil (viscosity between about 1,000 and 10,000 cp @ reservoir temperature), these terms will be used interchangeably herein. These tar sands may extend for many miles and occur in varying thicknesses of up to more than 300 feet. Although these deposits may lie at or near the earth's surface, generally they are located under a substantial overburden which may be as great as several thousand feet thick. Tar sands located at these depths constitute some of the world's largest presently known petroleum deposits. The tar sands contain a viscous hydrocarbon material, commonly referred to as bitumen, in an amount which ranges from 5 to about 20% by weight. Bitumen is normally immobile at typical reservoir temperatures. For example, in the Cold Lake region of Alberta, at a typical reservoir temperature of about 55.degree. F., bitumen is immobile with a viscosity exceeding several thousand poises. However, at higher temperatures, such as temperatures exceeding 200.degree. F., the bitumen generally becomes mobile with a viscosity of less than 345 centipoises.
Since most tar sand deposits are too deep to be mined economically, various in situ recovery processes have been proposed for separating the bitumen from the sand in the formation itself and producing the bitumen through a well drilled into the deposit. Among the various methods for in situ recovery of bitumen from tar sands, processes which involve the injection of steam are generally regarded as most economical and efficient. Steam can be utilized to heat and fluidize the immobile bitumen and, in some cases to drive the mobilized bitumen towards production means.
The most common and proven method for recovering viscous hydrocarbon is by using steam stimulation techniques which involve heating a formation in the vicinity of a well to stimulate production back through the same well. In this type of process, steam is injected into a formation by means of a well and the well is shut-in to permit the steam to heat the bitumen, thereby reducing its viscosity. Subsequently, all formation fluids, including mobilized bitumen, water and steam, are produced from the same well using accumulated reservoir pressure as the driving force for production.
The primary objective of a steam stimulation process is to transfer thermal energy into the formation and permit the rock to act as a heat exchanger. This heat then lowers the viscosity of the oil flowing through the heated volume. Normally, water-oil ratios are extremely high when the well is first returned to production, but the amount of water (and steam) produced declines and the oil production rate passes through a maximum that is usually much higher than the original rate.
Initially, sufficient pressure may be available in the vicinity of the wellbore to lift fluids to the surface; as the pressure falls, artificial lifting methods are normally employed. Production is terminated when no longer economical and steam is injected again. This cycle is then repeated many times until oil production is no longer economical.
During the early cycles of steam injection and production, oil production rates may be quite high since the oil nearest to the well is being produced. However, during subsequent steam cycles as the oil nearest the well is depleted, steam must move farther into the formation to contact the oil and as a result increased heat losses make the steam less effective as an oil recovery agent. The process loses efficiency with each cycle and eventually oil production becomes uneconomic. This is often simply because it costs more to generate the steam than any additional oil recovered in a cycle.
In steam stimulation processes the highest pressures and temperatures exist in the vicinity of the well immediately following the injection phase. Normally, this pressure and temperature will correspond to the properties of the steam which was employed. Before oil can be moved from the remote parts of the reservoir to the well, the pressure in the near well regions must fall so that it is lower than the distant reservoir pressure. During this initial depressurizing phase, the near wellbore reservoir material cools down as water flashes into steam. As mentioned, the first production from the well thus tends to be steam and this tends to be followed by hot water. Eventually, the pressure is low enough and oil can move to the wellbore.
In conventional steam stimulation processes, during the initial production phase, much of the heat which was put into the reservoir with the steam is simply removed again and is lost. Thus, a major inefficiency of thermal stimulation processes is that this heat must be supplied during each cycle and as the available oil becomes more remote from the well, the cyclic wasted heat quantity increases.
Nevertheless, the only methods which have been proven to be effective commercially in a wide range of reservoirs are steam stimulation processes, and these processes only recover a small portion of the bitumen with rapidly declining effectiveness following each steam injection/production cycle. A continuing need exists for a steam stimulation process which will more efficiently recover oil and reduce the amount of wasted thermal energy.